This invention relates to an image forming process including the step of developing electrostatic latent images formed in electrophotography, electrostatic recording, electrostatic printing, etc. by use of an organic photoconductive photosensitive member, with a two-component developer.
At present, for formation of a visible image from a certain image information, a method via electrostatic latent images such as electrophotography, etc has been widely used. For example, according to an example of electrophotography, electrostatic latent images formed on a latent image bearing member comprising a photoconductive photosensitive member by the charging step and the exposure step is developed with a developer comprising electroscopic colored particles called toner, and the resulting toner image is ordinarily transferred onto a transfer material and fixed to give a visible image.
The developers to be used in development of such electrostatic latent images may be classified into the so called two-component developer comprising a mixture of toner and carrier and the so called one-component developer comprising a magnetic toner containing magnetic material which is to be used solely without being mixed with carrier. In the system employing the two-component developer, toner is subjected to triboelectric charging by stirring mechanically toner with carrier, whereby it is possible to control to a considerable extent the polarity of charging and the amount of charging of the toner by choosing the characteristics of the carrier, the conditions of stirring, etc. In this respect, the two-component developer is superior to the one-component developer.
The developing method includes the magnetic brush method, the cascade method, etc., of which the magnetic brush has preferably been employed. The magnetic brush method is a method, in which pikes of developer erected in a brush-like shape by magnetic force on a developer conveying support, namely, magnetic brush, is formed and the magnetic brush is brushed against the surface of a latent image bearing member, thereby attaching toner particles onto electrostatic latent images to effect development.
In the developing method utilizing such a magnetic brush method, etc., the toner particles may also be attached on the background portion other than the electrostatic image portions on which they are to be attached, thereby staining the image and causing the problem of generation of the so called "fog". Such generation of fog is caused primarily by the toner which contains partially toner particles inadequately charged and will frequently occur in the case of, for example, containing weakly charged toner not reaching the amount of charging essentially required for toner, or oppositely charged toner having the polarity opposite to that essentially required for toner. For prevention of such generation of fog, a number of charge controlling techniques have been proposed in the prior art. Further, for example, application of an appropriate bias voltage between the developer conveying support and the latent image bearing member upon development has been used as an effective means.
However, when an organic photoconductive photosensitive member (herein after called "OPC photosensitive member") is used as the photoconductive member constituting the latent image bearing member, it has been found that there is generation of fog which cannot be removed even by application of a bias voltage. The cause for generation of such a fog is not necessarily clear, but it may be considered that the toner will be charged through friction with the OPC photosensitive member when the magnetic brushes the latent image bearing member and the toner will be attached on the background portion other than the latent image portion on the OPC photosensitive member, and the attached toner will be consequently transferred and fixed onto a transfer material whereby fog as mentioned above will be formed.
Also, toner in a developer in a developing processor is consumed every time with development and fresh toner is supplemented corresponding thereto. Thus, toner is constantly renewed in a developing processor. Whereas, as for carrier, the originally prepared carrier is used repeatedly over a long term. For this reason, after repeated development, characteristics of the carrier will be deteriorated and the triboelectric charging characteristic of the carrier becomes unstable to result in generation of fog through the increase of weakly charged toner or oppositely charged toner, whereby it is not consequently possible to obtain stably good images for a large number of times.
The present inventors have made intensive studies on the basis of such viewpoints and, as a result, found that generation of fog as described above depends seriously on magnitude of the impact force which an OPC photosensitive member receives from a magnetic brush when the magnetic brush brushes the OPC photosensitive member, and accomplished this invention on the basis of such a finding.